Blender 2.8 subdivision creating small bumps
The cause of this most likely is because of overlapping faces or your normals are on the wrong side. Meaning that the physical plane of your mesh is on the inside of your mesh and building it around that somewhere caused overlapping faces. You can see if your normals are on the wrong side by going into the material viewer on the right side of the view port render shading option, which is the down button next to the transparent globe and view the backface culling status there by checking the backface culling option.